Wire electric discharge machining (WEDM), is an extremely versatile and accurate machining process. A desired contour is cut in a workpiece by means of a wire electrode (wire). Wire and workpiece are moved relatively to each other according the instructions of a machining program. The workpiece material is removed by the action of electrical discharge pulses applied between wire and workpiece.
High-End WEDM achieve shape accuracy below 1 μm and surface roughness below Ra 0.05 μm. These results are achieved by releasing discharge pulses of very low energy. The overall machining accuracy is determined by the sum of all error components. It is desirable that errors which were tolerated in the past should behenceforth taken into account. In WEDMing the engagement of tool and workpiece, disregarding drag error, oscillations of the wire, wire wear, etc., is essentially linear.
With certain workpiece materials, working conditions, etc., it may happen that the wire leaves some cutting marks extending vertically on the machined surface. The cutting marks occur in the form of protruding ridges and grooves. There are various causes for the occurrence of cutting marks. Sometimes the protruding ridges are due to inhomogeneities of the workpiece material, in particular inclusions, f.i. slag particles, oxides, etc, being much less conductive than the base material.
As shown in FIG. 1, local material defects cause remarkable isolated protruding ridges. Sometimes, the cutting marks are caused by sudden detachment of abrasion debris, such as the comparably soft wire coating material accumulated at a upper current supply or at a wire guide. Due to these local defects the wire may deviate laterally by a small amount, but large enough to leave a cutting mark in the wire traveling direction. Sometimes the cutting marks are visible to the naked eye, and are therefore highly undesirable. Stopping and restarting of the WEDM process, f.i. in case of electrical power outage, is another cause of cutting marks.
Difficult working conditions or improper settings are other causes of cutting marks. Here, in most cases the protruding ridges and grooves are present over the whole machining surface or over an entire side of the workpiece, as shown in FIG. 2. These cutting marks are much smaller than the mostly isolated protruding ridges caused by material defects. The energy of the discharge pulses is lowered in each successive cut, so that it is sometimes difficult to remove cutting mark with conventional methods.
DE 42 22 186 C2 of BÜHLER suggests to slightly tilt the wire in the cutting direction when producing finishing cuts along straight or only slightly curved sections (see also U.S. Pat. No. 5,438,178 family member of BÜHLER, e.g. FIG. 4; description, col.5, ln.67-col.6, ln.22; claim 4). One of the upper and the lower wire guide anticipates the other wire guide, which causes a slight lag between an upper and a lower machining path. Since WEDM is often executed in a main and one or more trim cuts, the wire electrode is guided along the machining path under varying angles in consecutive cuts. In this way, cutting marks are at least partially removed—surface smoothing is achieved.
Also, BÜHLER provides information about the shifting amount between upper and lower wire guide, so as to obtain a good smoothing. However, BÜHLER does not disclosure how and under which circumstances the method could be implemented in practice, and in particular how it could be implement in corners and arc sections of large curvature.
An object of the present invention is to provide an improved method for controlling a WEDM process and an improved wire electric discharge machining apparatus.